The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, a pulse width modulation (PWM) method is a pulse modulation method and refers to a method of changing the width of a pulse according to the level of a modulation signal and performing modulation. The width of the pulse increases when the amplitude of a signal is large and decreases when the amplitude of the signal is small. However, the location or amplitude of the pulse is not changed. The PWM signal may control the speed of the DC motor, that is, current. The speed of the DC motor may be controlled using a variable resistor. In this case, if only about 50% of electricity of a battery is controlled and supplied to the motor, the remaining electricity of about 50% is consumed as heat loss of a resistor. Therefore, a PWM control method is preferred.
A PWM control method, more particularly, a half-bridge control method has a low power loss (several tens of Watts) and good efficiency as compared to an existing linear control method and thus may improve fuel efficiency of a vehicle. For this reason, recently, the control method of the DC motor has gradually changed to a half-bridge control method.
In PWM control, a duty ratio of a predetermined time T1 when a switch is turned on to a predetermined time T2 when the switch is turned off is controlled. The time T1 is controlled based on the voltage of an external source in a range from 0 to 100%.
In this manner, if power is supplied to the motor by variable percentage while current flowing in the DC motor is periodically changed, power of 100% is instantaneously supplied to the motor. However, since a frequency is about 20 kHz, one period is very short and thus a response speed of the motor has certain limits.
Since output corresponding to an average of T1 is constant and only the electronic switch is repeatedly turned on/off, the speed of the motor can be controlled. Specifically, it will cause only heat loss of an internal resistor of a semiconductor without heat loss of a variable resistor. Accordingly, control efficiency of a microcomputer becomes 95% or greater.
However, in controlling the current of the motor, since a DC motor uses high current, overcurrent of a reference value may flow to destroy a system when short-circuit or stall occurs. In the worst case, the winding of the motor is overheated due to overcurrent, thereby causing a fire.
In the related art, a DC motor controller of a vehicle is configured as shown in FIG. 1. A PWM control module shown in FIG. 1 includes a microcomputer 11 for converting a PWM signal received from an external controller into a digital signal for controlling a DC motor, a half bridge 12 including two FETs to control switching of the DC motor 10, a shunt resistor 17 for measuring the amount of current of the DC motor 10, a differential amplifier 18 for amplifying the amount of current and a regulator 14 for performing a reset operation when an error occurs in the microcomputer 11.
The PWM signal received from the external controller and converted into the signal sensible by the microcomputer is inputted to the microcomputer 11. The microcomputer 11 converts the converted PWM input signal into the digital signal for controlling the DC motor 10.
This digital signal is converted into a signal having a voltage value and a current value for driving the half bridge 12. A pair of FETs drives the DC motor 10 in one direction.
The shunt resistor 17 is used to compare an input PWM duty cycle and an output PWM duty cycle using the amount of current flowing in the motor 10. If a difference between the input duty cycle and the output duty cycle is greater than a predetermined threshold, a problem may occur in the microcomputer 11 or the DC motor may stall. If the DC motor 10 stalls, the microcomputer 11 detects stall and sets the duty cycle to 0% to prevent a fire from being caused in the DC motor 10.
However, the resistance value of the shunt resistor 17 used in the PWM control module in order to reduce influence on the driving current of the motor is very small and a resistor having a very small error rate should be used in order to reduce an error of a current measurement value. In addition, even when a resistance value is small, current of 10 to 20 ampere generally flows, a resistance element of several watts should be used.
In addition, since a voltage applied to the resistor is significantly small, a voltage difference across the resistor is as small as several tens of mV. Accordingly, the differential amplifier 18 capable of amplifying the voltage to several tens of times or more is used to amplify the voltage to a value detectable by the microcomputer 11.
However, since the differential amplifier and the comparator should be used to detect overcurrent of the DC motor, delay of the differential amplifier according to frequency and change in circuit constant of the comparator circuit may occur. Therefore, the circuit becomes complicated and costs may increase.